Sidewall passivation for HEMT devices

ABSTRACT

Some embodiments of the present disclosure relate to a high electron mobility transistor (HEMT) which includes a heterojunction structure arranged over a semiconductor substrate. The heterojunction structure includes a binary III/V semiconductor layer made of a first III-nitride material to act as a channel region of the e-HEMT, and a ternary III/V semiconductor layer arranged over the binary III/V semiconductor layer and made of a second III-nitride material to act as a barrier layer. Source and drain regions are arranged over the ternary III/V semiconductor layer and are spaced apart laterally from one another. A gate structure is arranged over the heterojunction structure and is arranged between the source and drain regions. The gate structure is made of a third III-nitride material. A first passivation layer is disposed about sidewalls of the gate structure and is made of a fourth III-nitride material.

REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. application Ser. No.15/234,590 filed on Aug. 11, 2016, which is a Divisional of U.S.application Ser. No. 14/488,380 filed on Sep. 17, 2014 (now U.S. Pat.No. 9,425,301 issued on Aug. 23, 2016), which claims priority to U.S.Provisional Application No. 61/986,389 filed on Apr. 30, 2014. Thecontents of the above-referenced Applications are hereby incorporated byreference in their entirety.

BACKGROUND

High-electron-mobility transistors (HEMTs), also known asheterostructure FETs (HFETs) or modulation-doped FETs (MODFETs), are atype of field-effect transistor. Whereas a traditional n-type MOSFETincludes a gate electrode arranged over a p-type doped channel regionthat separates n-type source/drain regions, for example, a HEMT deviceuses a heterojunction as the channel instead of a doped region. Thisheterojunction is defined by an interface at which two materials withdifferent band gaps meet one another. III-N (tri nitride) devices areone type of HEMT where the heterojunction is made up of a Group-IIImaterial (e.g., Al, Ga, In) and a nitride (N) material. These III-Ndevices show very promising performance in high-power and high-frequencyapplications. III-N devices can be used, for example, in high power-highfrequency applications such as emitters for cell phone base stations,Direct Broadcast Satellite (DBS) receivers, electronic warfare systems,etc.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 shows some embodiments of a cross sectional view of a HEMT devicein accordance with some embodiments of the present disclosure.

FIG. 2 shows a flow diagram of a method of manufacturing an e-HEMTdevice in accordance with some embodiments of the present disclosure.

FIGS. 3-11 show a series of cross-sectional views that collectivelydepict a method of manufacturing a HEMT device in accordance with someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of this disclosure.Specific examples of components and arrangements are described below tosimplify the present disclosure. These are, of course, merely examplesand are not intended to be limiting. For example, the formation of afirst feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

HEMT devices use a heterojunction between two materials with differentband gaps as a channel. For example, in some III-N HEMT devices, a widebandgap AlGaN layer can form a heterojunction with a narrow bandgap GaNlayer. The lattice constants of these two materials are typicallyslightly different. The difference in the lattice structures of thesetypes of materials produces a strain that can result inpizeoelectricity-induced polarization and form band bending at theheterojunction interface. For example, a GaN HEMT typically has a strongsurface polarization that causes it to operate with normally-on(depletion-mode) status. To overcome the surface polarization andcontrol the flow of charge carriers in an enhancement mode device, agate having a p-GaN material with a high work function can be formeddirectly on top of the AlGaN layer.

However, the GaN HEMT with an AlGaN/GaN as the schottky barrier and ap-GaN gate as the control gate results in a large gate leakage. Besides,in order to form an enhancement mode device structure, lots of III-Nsurfaces suffered process-induced traps or damage. These traps or damagemainly contribute gate leakage or device leakage current which causesdevice performance to degrade. To attempt to limit the number of traps(and thereby improve device performance), the present disclosure setsforth techniques where a passivation layer is formed on III-N surface,for example sidewalls of the p-GaN. This passivation layer terminatesand passivates dangling bonds on the surface of gate sidewall surfacesto limit the number of interfacial traps and thereby helps to improvedevice performance. Thus, gate leakage can be reduced by introducingthis passivation layer.

FIG. 1 shows some embodiments of a cross sectional view of a HEMT device100 in accordance with the present disclosure. The HEMT device 100includes a heterojunction structure 102 arranged over a semiconductorsubstrate 104. The heterojunction structure 102 is made up of a binaryIII/V semiconductor layer 106, and a ternary III/V semiconductor layer108 arranged over the binary III/V semiconductor layer 106. The binaryIII/V semiconductor layer 106 is made of a first III-nitride materialand acts as a channel region of the e-HEMT. The ternary III/Vsemiconductor layer 108 is made of a second III-nitride layer and actsas a barrier layer, somewhat analogous to a gate dielectric for aconventional MOSFET. In some embodiments, the binary III/V semiconductorlayer 106 is made of gallium nitride (GaN), and the ternary III/Vsemiconductor layer 108 is made of Aluminum Galium Nitride(Al_(x)Ga_(1-x)N, where 0<×<1).

One or more buffer layers 110 can be arranged between the heterojunctionstructure 102 and the substrate 104. These buffer layers 110 can helpgradually distribute strain over their thickness, wherein the strain isdue to lattice mismatch between the substrate 104 and the binary III/Vlayer 106. By distributing strain, these buffer layers 110 can help wardoff trap formation in some regards. The illustrated buffer layers 110include an uppermost buffer layer 112 abutting the binary III/V layer106, as well as a lower buffer layer 114 between the upper buffer layer112 and the substrate 104. In some embodiments, the uppermost bufferlayer 112 can be made of AlGaN, and the lower buffer layer 114 can bemade of AlN. In other embodiments, more than two buffer layers can beincluded between the heterojunction structure 102 and substrate 104.

Conductive source and drain regions 116, 118 are arranged over theternary III/V semiconductor layer 108 and spaced apart laterally fromone another. The conductive source and drain regions 116, 118 haverespective lower regions which abut the ternary III/V semiconductorlayer 108 and which are ohmically coupled to the ternary III/Vsemiconductor layer 108. In some embodiments, the source/drain regions116, 118 rest directly on and abut the ternary III/V semiconductor layer108 and are spaced apart from the binary III/V semiconductor layer 106.However, in other embodiments, the source/drain regions 116, 118 extendthrough the ternary III/V semiconductor layer 108 and abut the binaryIII/V semiconductor layer 106.

A gate structure 120 is arranged over the heterojunction structure 102and is arranged laterally between the conductive source and drainregions 116, 118. The gate structure 120 is made of a third III-nitridematerial. For example, in some embodiments, the gate structure 120 canbe made of GaN, which has been doped with donor impurities to form ann-type gate structure or which has been doped with acceptor impuritiesto form a p-type gate structure. These dopants help to make theresultant HEMT device 100 operate in enhancement mode as opposed todepletion mode.

A first conformal passivation layer 122 is disposed about gate structuresidewalls 120A, 120B and over an upper gate structure surface 120C. Thefirst conformal passivation layer 122 is also disposed over an uppersurface 108A of the ternary III/V barrier semiconductor layer 108. Thisfirst conformal passivation layer 122, which can be made of aluminumnitride (AlN) or boron nitride (BN) in some embodiments, can be a highquality thin film to prevent current leakage from the gate structure120. Thus, in some embodiments, the first conformal passivation layer122 is grown by atomic layer deposition (ALD) techniques, which althoughtime-intensive, produce a very high quality film. The first conformalpassivation layer 122 can have a thickness of between approximately 5angstroms and approximately 500 angstroms in some embodiments. Inaddition to providing a high quality film, ALD techniques areadvantageous because they can be carried out at relatively lowtemperature, for example between 200° C. and 500° C., which helps limitthermal budget issues, and because ALD techniques provide good stepcoverage compared to PVD.

A second conformal passivation layer 124 is disposed over the firstconformal passivation layer 122. This second conformal passivation layer124 can help protect the first conformal passivation layer 122 duringprocessing. In some embodiments, this second conformal passivation layer124 can be made of a nitride (e.g., SiN) or an oxide (e.g., SiO2). Thesecond conformal passivation layer 124 can have a thickness that isgreater than that of the first conformal passivation layer 122 and canbe formed by a technique that is different from that used to form thefirst conformal passivation layer 122. For example, in some embodiments,the second conformal passivation layer 124 can have a thickness ofapproximately 50 nanometers to approximately 500 nanometers. Further, insome embodiments, the second conformal passivation layer 124 can beformed by chemical vapor deposition (CVD), plasma-enhanced chemicalvapor deposition (PECVD), or plasma vapor deposition (PVD), for example.

The conductive source/drain regions 116, 118 extend downward through thefirst and second passivation layers 122, 124 to ohmically couple to theternary III/V semiconductor layer 108. The conductive source/drainregions 116, 118 can be metal, such as copper, aluminum, tungsten,nickel, iron, cobalt, silver, gold, or platinum, for example.

A conformal dielectric capping layer 126 overlies the conductivesource/drain regions 116, 118. In some embodiments, this conformaldielectric capping layer 126 is a nitride (e.g., SiN) or an oxide (e.g.,SiO2), for example. In some embodiments, the conformal dielectriccapping layer 126 has a thickness that is greater than or equal to thethickness of second conformal passivation layer 124.

A metal electrode liner or metal electrode contact 128 extends downwardthrough the dielectric capping layer 126 and through the first andsecond passivation layers 122, 124 to form an ohmic connection to thegate structure 120. In some embodiments the metal electrode liner ormetal electrode contact 128 is formed by PVD or CVD. The metal electrodeliner or metal electrode contact 128 can extend downwardly alongsidewalls of the capping layer 126 and of the first and secondpassivation layers 122, 124 before ohmically contacting an upper surfaceregion of the gate structure 120.

Due to a difference in bandgaps between the binary III/V layer 106 andternary III/V layer 108, highly mobile charge carriers in the form of atwo-dimensional electron gas (2DEG) are established at the interfacebetween layers 106, 108. Hence, during operation, a voltage applied tothe gate electrode 120 controls the number of carriers (e.g., 2DEG) thatcan flow from the source 116 to the drain 118, or vice versa, through achannel region in layer 106. Thus, whether the HEMT 100 is in aconductive or resistive state can be controlled by controlling the 2DEGwith the help of the gate electrode 120. In many cases the HEMT device100 is an enhancement mode device, which operate similar to siliconMOSFET devices by normally being in a non-conducting state (normallyoff). Due to the nature of the heterojunction interface between 106/108,and the formation of the 2DEG at this heterojunction interface in HEMTs,such devices that are formed in the III-N materials system tend to benormally on, or depletion mode devices. i.e.; the high electron mobilityof the 2DEG at the interface of the AlGaN/GaN layers permits the III-Ndevice, such as a HEMT device, to conduct without the application of agate potential.

In the off-state, traditional enhancement mode HEMT (e-HEMT) devices canexhibit current leakage from their gate structure. The passivation layer122 can help limit this current leakage by approximately an order ofmagnitude in some instances.

FIG. 2 illustrates a flow diagram of some embodiments of a method ofmanufacturing a HEMT device according to some embodiments of thedisclosure. While method is illustrated and described below as a seriesof acts or events, it will be appreciated that the illustrated orderingof such acts or events are not to be interpreted in a limiting sense.For example, some acts may occur in different orders and/or concurrentlywith other acts or events apart from those illustrated and/or describedherein. In addition, not all illustrated acts may be required toimplement one or more aspects or embodiments of the description herein.Further, one or more of the acts depicted herein may be carried out inone or more separate acts and/or phases.

In 202, a first buffer layer made of a first III-nitride material isformed over a substrate. In 204, a second buffer layer is formed overthe first buffer layer, wherein the second buffer layer is made of asecond III-nitride material that is different from the first II-nitridematerial. In 206, a binary III-nitride channel layer is formed over thesecond buffer layer. In 208, a ternary III-nitride barrier layer isformed over the binary III-nitride channel layer. The ternaryIII-nitride barrier layer meets the binary III-nitride channel layer ata heterojunction interface. At 210, a binary III-nitride gate layer isformed over the ternary III-nitride barrier layer and is doped withdonor or acceptor impurities. At 212, selected portions of the dopedbinary III-nitride gate layer are removed to form a patterned dopedbinary III-nitride gate structure having an upper gate surface and outergate sidewalls. Removal of these selected portions leaves an uppersurface region of the ternary III-nitride barrier layer exposed. At 214,a first conformal passivation layer is formed over the upper gatesurface, outer gate sidewalls, and exposed upper surface region of theternary III-nitride barrier layer. This first conformal passivationlayer can help to reduce gate sidewall traps, and therefore helps limitcurrent leakage from the gate structure during operation of the device.

Turning now to FIGS. 3-11, one can see a series of cross-sectional viewsthat collectively depict formation of a HEMT device in accordance withsome embodiments. It will be appreciated that although specificstructural features are disclosed in these cross-sectional views, thatthese specific structural features are not necessarily required in allimplementations.

FIG. 3 is consistent with some embodiments of a structure formed byreference numerals 202-210 of FIG. 2. The structure of FIG. 3 includes asubstrate 302, which can take a variety of different forms. In someembodiments, the substrate 302 is a silicon substrate, a silicon carbide(SiC) substrate, or a sapphire substrate. A first buffer layer 304,which may in some instances be referred to as a lower buffer layer, isthen formed over the substrate 302, for example by epitaxial growthtechniques. In some instances, the first buffer layer 304 is an aluminumnitride (AlN) layer. A second buffer layer 306, which in some instancesmay be referred to as an upper-most buffer layer, is then formed overthe first buffer layer 304, for example by epitaxial growth techniques.In some instances, the second buffer layer 306 is an AlGaN layer. Abinary III-nitride channel layer 308 is then formed over the secondbuffer layer 306, for example by epitaxial growth techniques. In someinstances, the binary III-nitride channel layer 308 is a GaN layer. Aternary III-nitride barrier layer 310 is formed over the binaryIII-nitride channel layer 308, for example by epitaxial growthtechniques. In some embodiments, the ternary III-nitride barrier layer310 is an Al_(x)Ga_(1-x)N layer, wherein 0<×<1. A binary III-nitridegate layer 312 is formed over the ternary III-nitride barrier layer 310,for example by epitaxial growth techniques and is doped with donor oracceptor impurities. In some embodiments, the binary III-nitride gatelayer 312 is an n-type or p-type GaN layer.

In FIG. 4, a gate structure mask layer is formed and patterned over thebinary III-nitride gate layer. The gate structure mask layer can be aphotoresist layer, a hardmask layer, such as a nitride layer, and/orother individual layers or combinations of layers. With the patternedgate structure mask 402 in place, an etch is performed to selectivelyremove exposed portions of the binary III-nitride gate layer 312,thereby leaving a gate structure 312′ with a gate structure mask 402thereover. The patterned gate structure mask 402 is then removed.

In FIG. 5, a first conformal passivation layer 502 is formed by atomiclayer deposition (ALD). In some embodiments, the first conformalpassivation layer 502 is AlN or BN, and is deposited to a thickness ofbetween approximately 5 angstroms and 500 angstroms. To limit gatesidewall traps, the first conformal passivation layer 502 directly abutssidewalls and upper surface of gate electrode 312′.

In FIG. 6, a second conformal passivation layer 602 is formed over thefirst conformal passivation layer 502 to protect the first conformalpassivation layer 502 during processing. In some embodiments, the secondconformal passivation layer is a nitride, such as SiN for example, or anoxide, such as SiO2 for example. In some embodiments, the secondconformal passivation layer 602 is formed by a different technique thanthe first conformal passivation layer 502. For example, the secondconformal passivation layer 602 can be formed by CVD, PECVD, or PVD,which has a deposition rate that is faster than that of ALD to maintainprocess throughput at good levels. To help protect the first conformalpassivation layer 502, the thickness of the second passivation layer 602can range between approximately 50 nm and approximately 500 nm.

In FIG. 7, a source/drain mask 700 has been formed over the secondconformal passivation layer. With the source/drain mask 700 in place, anetch, such as a dry etch for example, is performed to form source/drainopenings 702 which extend through the first and second conformalpassivation layers 502, 602 and which terminate on the ternaryIII-nitride barrier layer 310. Some portions of the ternary III-nitridebarrier layer 310 can be removed/consumed during this etch, but otherportions can be left under the source/drain openings 702 above thebinary III-nitride channel layer 310. In FIG. 8, the source/drain mask700 is removed, and the source/drain openings are filled with aconductive material, such as a metal. The metal when initially formedextends over the entire exposed surface of the second conformalpassivation layer. Subsequently, a mask (not shown), such as aphotoresist mask, is formed over the source/drain regions, and an etch,such as a dry etch, is carried out to form the illustrated conductivesource/drain regions 802.

In FIG. 9, a conformal dielectric capping layer 902 is formed. In someembodiments, the conformal dielectric capping layer is a nitride, suchas SiN for example, or an oxide, such as SiO2 for example.

In FIG. 10, a gate electrode mask 1000 is formed over the dielectriccapping layer. With the gate electrode mask in place, an etch, such as adry etch, is performed to form a gate electrode opening 1002. The gateelectrode opening extends through the dielectric capping layer, thefirst conformal passivation layer, and a second conformal passivationlayer. The gate electrode opening terminates on the patterned dopedbinary III-nitride gate structure.

In FIG. 11, a conductive gate electrode liner 1100 is formed in the gateelectrode opening. In some embodiments, the conductive gate electrodeliner is deposited by PVD or CVD. The conductive gate electrode layercan comprise a metal, such as aluminum, copper, tungsten, or nickel, forexample, or can include other conductive materials, such as dopedpolysilicon for example.

As can be appreciated from the foregoing, the present disclosure setsforth techniques whereby a passivation layer is formed over sidewalls ofthe gate electrode to limit interfacial traps. This passivation layerterminates and passivates dangling bonds on the surface of gate sidewallsurfaces to limit the number of interfacial traps and helps to improvedevice performance. In particular, this passivation layer reduces gateleakage current.

Therefore, some embodiments of the present disclosure relate to a highelectron mobility transistor (HEMT) which includes a heterojunctionstructure arranged over a semiconductor substrate. The heterojunctionstructure includes a binary III/V semiconductor layer made of a firstIII-nitride material to act as a channel region of the e-HEMT, and aternary III/V semiconductor layer arranged over the binary III/Vsemiconductor layer and made of a second III-nitride material to act asa barrier layer. Source and drain regions are arranged over the ternaryIII/V semiconductor layer and are spaced apart laterally from oneanother. A gate structure is arranged over the heterojunction structureand is arranged between the source and drain regions. The gate structureis made of a third III-nitride material. A first conformal passivationlayer is disposed about sidewalls of the gate structure and is made of afourth III-nitride material.

Other embodiments of the present disclosure relate to a method offorming an enhancement mode, high electron mobility transistor (e-HEMT)on a substrate. In this method, a binary III-nitride channel layer isformed over the substrate. A ternary III-nitride barrier layer is formedover the binary III-nitride channel layer. The ternary III-nitridebarrier layer meets the binary III-nitride channel layer at aheterojunction interface. A binary III-nitride gate layer is formed overthe ternary III-nitride barrier layer and is doped with donor oracceptor impurities. Selected portions of the doped binary III-nitridegate layer are removed to form a patterned doped binary III-nitride gatestructure having an upper gate surface and outer gate sidewalls.Removing the selected portions of the gate layer leaves an upper surfaceregion of the ternary III-nitride barrier layer exposed. A firstconformal passivation layer is formed over the upper gate surface, outergate sidewalls, and exposed upper surface region of the ternaryIII-nitride barrier layer.

Still other embodiments relate to a high electron mobility transistor(HEMT) formed on a substrate. The HEMT includes an AlN buffer layer overthe substrate. An AlGaN buffer layer is arranged over the AlN bufferlayer. A GaN channel layer is arranged over the AlGaN buffer layer. AnAlGaN barrier layer is arranged over the GaN channel layer. A GaN gatestructure is arranged over the AlGaN barrier layer. The GaN gatestructure is doped with acceptor or donor impurities and has a gatestructure upper surface and gate structure outer sidewalls. An AlN or BNconformal passivation layer is arranged over the gate structure uppersurface and abuts the gate structure outer sidewalls.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A high electron mobility transistor (HEMT),comprising: a heterojunction structure arranged over a semiconductorsubstrate, the heterojunction structure comprising: a binary III/Vsemiconductor layer made of a first III-nitride material, and a ternaryIII/V semiconductor layer arranged over the binary III/V semiconductorlayer and made of a second III-nitride material; source and drainregions arranged over an upper surface of the ternary III/Vsemiconductor layer and spaced apart laterally from one another; a gatestructure arranged over the heterojunction structure and arrangedbetween the source and drain regions, wherein the gate structure isarranged over the upper surface of the ternary III/V semiconductorlayer; a first passivation layer disposed about sidewalls of the gatestructure and extending over an upper surface of the gate structure,wherein the first passivation layer is made of a fourth III-nitridematerial; and a second passivation layer overlying the first passivationlayer, wherein a material composition of the second passivation layer isdifferent from a material composition of the first passivation layer. 2.The HEMT of claim 1, wherein the fourth III-nitride material of thefirst passivation layer is a binary III/V semiconductor material havingdifferent binary semiconductor constituents from the first and secondIII-nitride materials.
 3. The HEMT of claim 1, wherein the gatestructure is doped n-type or p-type, and wherein the binary III/Vsemiconductor layer is an intrinsic semiconductor material.
 4. The HEMTof claim 1, wherein the first passivation layer is conformal and has athickness of between 5 Angstroms and 500 Angstroms.
 5. The HEMT of claim1, further comprising: one or more buffer layers under the binary III/Vsemiconductor layer, wherein an uppermost buffer layer is made of thesecond III-nitride material and wherein a lower buffer layer under theuppermost buffer layer is made of the first III-nitride material.
 6. TheHEMT of claim 5, wherein the first passivation layer is made of the samematerial as the lower buffer layer.
 7. The HEMT of claim 6, wherein: thefirst III-nitride material of the binary III/V semiconductor layer isGaN; the second III-nitride material of the ternary III/V semiconductorlayer is Al_(x)Ga_(1-x)N; the gate structure is made of a thirdIII-nitride material of n-type or p-type GaN; and the fourth III-nitridematerial of the first passivation layer is AlN or BN.
 8. The HEMT ofclaim 1, wherein the second passivation layer is conformally disposedalong outer sidewalls of the first passivation layer and over an uppersurface of the first passivation layer, wherein the second passivationlayer has a second thickness that is greater than a first thickness ofthe first passivation layer; a capping layer conformally disposed alongouter sidewalls of the second passivation layer and over an uppersurface region of the second passivation layer; and a metal gateelectrode including edges that overlie an upper surface region of thecapping layer, and including inner sidewalls that extend downward alongsidewalls of an opening through the capping layer, through the secondpassivation layer, and through the first passivation layer to be indirect electrical connection with an upper surface of the gatestructure.
 9. An enhancement mode high electron mobility transistor(HEMT) formed on a substrate, comprising: an AlN buffer layer over thesubstrate; an AlGaN buffer layer over the AlN buffer layer; a GaNchannel layer over the AlGaN buffer layer; an AlGaN barrier layer overthe GaN channel layer, wherein source and drain regions are over anupper planar surface of the AlGaN barrier layer; a GaN gate structure indirect contact with the upper planar surface of the AlGaN barrier layer,wherein the GaN gate structure is doped with acceptor or donorimpurities and has a gate structure upper surface and gate structureouter sidewalls; an AlN or BN passivation layer over the AlGaN barrierlayer and extending over the gate structure upper surface and abuttingthe gate structure outer sidewalls; and a second passivation layerdisposed along outer sidewalls of the AlN or BN passivation layer andover an upper surface of the AlN or BN passivation layer, wherein amaterial composition of the second passivation layer is different from amaterial composition of the AlN or BN passivation layer.
 10. The HEMT ofclaim 9, wherein the AlN or BN passivation layer is conformal and has athickness of between 5 Angstroms and 500 Angstroms.
 11. The HEMT ofclaim 9, wherein the source and drain regions extend through the AlN orBN passivation layer and through the second passivation layer andterminate on the AlGaN barrier layer.
 12. A high electron mobilitytransistor (HEMT), comprising: a heterojunction structure arranged overa semiconductor substrate, the heterojunction structure comprising: achannel layer comprising a first III-nitride material, and a barrierlayer comprising a second III-nitride material; source and drain regionsover an upper surface of the heterojunction structure and spaced apartlaterally from one another; a gate structure arranged over theheterojunction structure and arranged between the source and drainregions; a first passivation layer disposed about sidewalls and an uppersurface of the gate structure, and comprising a fourth III-nitridematerial; and a second passivation layer disposed over the firstpassivation layer; and wherein the first passivation layer has a firstthickness and the second passivation layer has a second thickness thatis greater than the first thickness.
 13. The HEMT of claim 12, whereinthe fourth III-nitride material of the first passivation layer is abinary III/V semiconductor material having different binarysemiconductor constituents from the first and second III-nitridematerials.
 14. The HEMT of claim 12, further comprising: one or morebuffer layers under the channel layer, wherein an uppermost buffer layeris made of the second III-nitride material and wherein a lower bufferlayer under the uppermost buffer layer is made of the first III-nitridematerial.
 15. The HEMT of claim 14, wherein the first passivation layerand the lower buffer layer are composed of same constituent materials.16. The HEMT of claim 12, wherein: the first III-nitride materialcomprises GaN; the second III-nitride material comprisesAl_(x)Ga_(1-x)N; and the fourth III-nitride material comprises AlN orBN.
 17. The HEMT of claim 12, wherein the source and drain regionsextend through the first and second passivation layers and terminate onthe barrier layer.
 18. The HEMT of claim 12, wherein the secondpassivation layer is conformally disposed along outer sidewalls of thefirst passivation layer and over an upper surface of the firstpassivation layer, further comprising: a capping layer conformallydisposed over an upper surface region of the second passivation layer;wherein the capping layer is conformally disposed along outer sidewallsof the second passivation layer and over an upper surface of the secondpassivation layer.
 19. The HEMT of claim 18, further comprising: a metalgate electrode including edges that overlie an upper surface region ofthe capping layer, and including inner sidewalls that extend downwardalong sidewalls of an opening through the capping layer, through thesecond passivation layer, and through the first passivation layer to bein direct electrical connection with an upper surface of the gatestructure.
 20. The HEMT of claim 12, wherein the first passivation layerhas a thickness of between 5 Angstroms and 500 Angstroms.